Thermionic amplifier



Patented Nov. 25, 1941 UNITED a? Price 2,264,197 THERIVHONIC ALIFIERware Application May 17, 1940, Serial No. 335,759 In Great Britain April17, 1939 4 Claims.

The present invention relates to thermionic amplifiers and ismoreparticularly concerned withamplifiers giving a balanced output withrespectto the battery busbars from an unbalanced input.

The object of the invention is to provide a thermionic amplifier givingabalanced output which iscapable of responding to an unbalanced input ofa steadysor an. alternating nature.

According therefore, to. the invention the amplifier: comprisestwo-thermionic valves and having;,a commoncathode resistance, the anodeof onevalve being connected over a direct current path. to the. grid ofthe second valve over a resistance. potentiometer of: such a value thatthe gainioi the-rfirstvalve-at the grid of the second ispunity'.

The-invention has-particular application in association with a cathoderay tube since it enables certain-well known defects of the tube to beovercome and? also'itprovides certain facilities which enable the tubeto beemployed with greater accuracy as a precision instrument.

As1is wellknown. there are two main types of cathode ray tube.-or--oscillograph in general use, one which depends on the presence ofa small amount of gasto securea good focus of the beam, and one whichis:substantially fully evacuated and securesiocusing action by theco-relation of potentials-on certain electrodes. The former isgenerally. known as a. gas-filled or soft tube and the-latterasahard-tube. Deflection. of the oathode ray. beam, generally. alongco-ordinate axes, may be accomplished with bothtypes of tube bymeansoi-internal. pairs. of plates or by external pairs. ofv coils of.wire. Although the invention is, mainly concernedv with, the former typeof tube,.it.may be applied equally well to the latter with certainadvantages tobe described later.

With both types of. tubes certain distortions occur when electrostaticdeflection is used, such distortion being .due. to variations in themean potential of; the deflecting plates, with respect to the final,anode of the system of focusing electrodes. In the case of the hard.tube progressive defocusing of the spot towards its extreme excursionsoccurs, whilst alteration of the effective deflection sensitivity givesrise to the well known trapezium distortion efiect. In the case of thegas-filled tube generally only the latter type of distortion occurs.It-has been proposed to overcome these defects by applying equal andopposite deflection voltages to the pairs of deflecting plates by meansof transformers or push pull thermionic valve stages but owing to thereactive couplings used with these methods deflection of the spot bydirect current voltages is impossible. Likewise means for adiusting theinitial position of the spot are generally only operative on alternatinginput.

In theapplicationof the invention to a cathode ray tube or oscillographone pair of the deflecting plates is directly connected one to the anodeof one valve and the other to the anode of the second valve.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawing which shows thecircuit of the amplifier arranged to provide equal and oppositepotentials to a pair of deflecting plates of a cathode ray. tube In thedrawing each of the pair of deflecting plates X-X (for, say,the-horizontal axis) is directlyconnected tothe anodes of a pair ofsimilar thermionic valves VI and V2which are preferably of the pentodetype. Theanodeimpedances of the pair of valvesconsist of. resistancesRI. and R2 which are nominally equal and of value deter mined in knownmannenfrom the characteristics of the valves when used foramplification. purposes. The anode-of the valve VI-is connected-to oneend of a high resistancepotentiometer- PI P2, the other endbeing-connected tothe negative busbar, whilst-the tap is connected tothegrid of the valve V2 over theresistanceGZ the purpose of which will bedescribed later. The potentiometer PI--P2 isof such a resistance thatthe ratio of the-change ofvoltage applied tothe grid of V2. to-thechangeof anode voltage of VI due to any input voltageisnominally. equal to.the inverse of the gain of. the valve VI, i. e. the gain ofthe valveVI. at the gridotthe valve V2 is unity, sothat fora given inputto thegrid of VI equal andopposite changes. of. voltage occur on the anodesand therefore on-the deflecting plates. The cathodes of the valvesVIandVZ are connected to twoequal resistances TI and r2, whose remainingendsare joined together and to a further variable resistance r3, whichterminates on the negative busbar. ,Thevcathoderesistances TI and T2 areof value derived in known manner for production of grid bias so that thevalves may amplify with a minimum of distortion. The voltage to beexamined, by deflection of the cathode ray spot, is applied over asuitable potentiometer R, GI to the grid of valve VI and the junctionofresistances rI, T2 and r3.

Since the gains, of the valves VI and V2 are made equal and produce.opposite changes of anode currents the voltage on r3 will be constantand is therefore used, by adjustment of the value of 1'3. to offset thepositive static bias on the grid of V2 derived from the anode of VI overthe coupling potentiometer. In order to produce no resultant voltagebetween the deflection plates under static conditions T3 is adjusted sothat the position of the spot does not alter when the cathodes of thevalves VI and V2 are joined. Since the anode and cathode resistances arenominally equal then it is known that the anode currents are alsonominally equal. In cases where the anode currents of the two valvesdifier widely when tested under similar conditions it is preferabl tohave the valve with the higher anode current as V2, so that the aboveadjustment puts a larger bias than normal on V2. If such a valve wereused as VI then V2 would be under-biased and would produce grid currentat a lower input voltage than is desirable.

It will be seen that the above circuit is both statically anddynamically balanced and has no appreciable reactive couplings so thatit can be used on direct or alternating input. The types of valve usedare not important but it is preferable to use pentode valves, when it ispossible to use a reduced screen voltage by connecting the screenstogether and to a suitable resistance S terminating on the positivebusbar. This is possible without unwanted coupling effects in view ofthe equalisation of anode currents (and hence to a first approximationof screen currents) and of the equal and opposite gains of each valve.

A further advantage of employing pentode valves is thatalteration of theinitial spot position without alteration of the mean potential betweenthe deflection plates can be secured by converting a portion of theanode resistances RI and R2 into a potentiometer withthe tapping pointtaken to the positive busbar as shown in the drawing. With the arms of.the potentiometer in the centre position conditions are as describedformerly but as the arm is moved towards the. anode of VI for instancethe latter is raised and the anode of V2 is lowered in potential byequal 7 amounts with respect to the positive busbar; the

spot being 'thereby moved towards the plate connectedto the anode of VI.pentode valve is very nearly proportional to the anode impedance'thegain of VI is thereby reducedwhilst that of V2 is increased but the overall ga n of the system will fall since V2 is fed However, the change inthe overall from VI. gain is not found to be material for the usualdegree of alteration of spot position normally reouired, such as may beobtained by making the total potentiometer resistance canal to one thirdof the total anode resistances (RI plus R2).

As is normal the approximate division of the busbar voltage betweenvalve and anode res stance with no input, will be equal, so that ineffect the maximum deflecting voltage available is some 80% of thebusbar Voltage. The latter is therefore determined by the full screendeflection voltage of the tube. The proportion of RI plus R to be usedas a shift control therefore is canal to desired range of control forinstance one-third. In applying this Valve to the circuit account mustbe taken of the fact that the maximum anode resistance w ll occur whenthe control is at ither end and this latter value will be determined bythe anode characteristics of the valve.

Taking this figure, the change of gain of the whole sta e as the shiftcontrol is moved from the centre to either end, w ll be one-sixth at themost. since the gain of V2 tends to be constant whilst that of VI altersby the change of anode impedance. It is only rarely that shifts of thisorder have to be used, and then the gain can be restored by the inputcontrol.

If pentode valves are used, as is preferred, owing to the constantcurrent characteristics of both screen and anode with changes in anodevoltage, the above described method of altering the position of the spotproduces very little change in the anode and screen currents. Hence thestatic grid bias conditions will not be altered,

Since the gain of a and the control will be equally effective on bothdirect and alternating inputs. In addition the mean potential of theplates remains unaltered with alteration of the shift control so thatneither defocusing or trapezium distortion occurs.

If triode valves are used these advantages cannot fully be obtainedsince for maximum gain (or maximum output voltage swing) the'anoderesistance must be much larger than the internal impedance of the valveand hence thestatic anode current will change almost in inverse proamplifier be just insufiicient for certain uses, for

instance when the input voltage is very small,

. the feedback action can be dispensed with to any desired degree byconnecting the cathodes together via a resistance 14 by'closing theswitch SW. This will introduce no change in the static conditions sincethis is the method whereby the amplifier is set up, but will reduce thefeedback on each valve. It is not found practicable to connect thecathodes by resistances less than some one third of either H or 12,since a form of instability can be set upin the amplifier due to thedirect couplings between the grid of V2, the anode ofVl, and thecathodes. A limited degree of gain control can be-obtained by making r4variable.

The cathodes may also be connected by a condenserof value chosen-to givereduced feedback at high input frequencies so as to enable the upperworking frequency limit'to be extended. In addition small inductancesmay be included in the anode impedances to give the same efiect in thewell known manner.

It has been mentioned that the voltage to be examined by deflection ofthe cathode ray spot is applied over a suitable potentiometer to thegrid of valve VI and the junction of resistances r! and T2. In practice,as is well known, the impedance of this input potentiometer is limitedby the interference voltages induced in it by external electricalapparatus. With a given value of input impedance however it is possibleto neutralise the effect of the pickup in the amplifier described byplacing an additional impedance G2 in the grid lead of V2. For instance,if the input impedance GI of VI be made say 100,000 ohms and the portionPI of the coupling potentiometer between VI and V2 applied to the gridof V2 is also 100,000 ohms, then the pickup on the latter will beneutralised by the pickup on the formerby being amplified by Valve VIand reduced to unity gain on the coupling potentiometer. Thus theinterference deflection of the spot will be due only to the anodevariations of VI. By introducing in the grid connection of V2 a furtherresistance G2 the consequent additional pickup on V2 can be made to beof substantially the same phase and magnitude as that on VI so that theresulting interference deflection of the spot will be zero. It istherefore proposed to use such co-related grid impedances for V! and V2and to provide for the main gain control of the input voltage by aseries variable resistance R, preferably of the inverse logarithmictype. It is to be noted however that if the pickup is too great thefocussing action may be upset. Further the earthy connection on theinput is taken from the junction of r! and T2 to prevent the voltage onT3 from appearing on R and GI when a low resistance path exists on theinput.

It will be appreciated that the mean voltage of the deflection plates isintermediate between that of the positive and negative busbars and thatin the interests of obtaining uniformly good focus over the Whole of thscreen, the final anode or gun of the cathode ray tube must be broughtto the same approximate potential. This may be readily achieved byconnection of the anode to a potentiometer connected across the busbars.Incidentally this method of connection can be arranged so as to supply apart of the normal anode voltage requirements of the tube.

With a typical circuit using H. F. pentodes the maximum anode resistancewas found to be 75,000 ohms, giving a shift potentiometer of 50,000 ohmsfor shift control. The cathode resistances TI and T2 were 1,000 ohms and13 was some 2,000 ohms. The stage gain was 50 giving an overall gain of100 which was found to be adequate for deflections of one volt, whilstwith 14 at 300 ohms and by closing switch SW the gain could be increasedby approximately three times without instability. Valves with commerciallimits of 30% can be accommodated by adjustment of T3, whilst pickup(mainly 50 cycles in this case) could be made imperceptible at any gainwhen Gl and P2 were 100,000 ohms and R and PI were megohms, by making G2200,000 ohms.

As mentioned previously the type of amplifier described may be used withmagnetic deflection of the cathode ray beam by replacing the anoderesistances or a part thereof by deflection coils. It will be seen thatthe static anode currents will now produce no permanent deflection ofthe beam, since the coils must be connected in magnetic opposition inorder to produce a magnetic deflection field from the opposite changesof anode currents due to an input voltage. Likewise the shift controlconsisting of a potentiometer which is part of the anode impedances canalso be used; Whilst the method of setting up the amplifier remainsunchanged.

Since the amplifiers may be constructed with commercial resistances andvalves, two such amplifiers may be accommodated in a small space aboutthe tube socket with the bulk of the couplings wired between the valveholder tags.

Although in the above description reference has only been made to onepair of deflection plates or coils the amplifier may be applied equallywell to the remaining co-ordinate pair of plates or coils. In this casearrangements can be made, if so desired, to offset the normaldifferences in deflection sensitivities of the two axes of the tube byan appropriate difference in the gains of the amplifiers.

It will be appreciated by those skilled in the art that the amplifierdescribed may be used for purposes other than in connection with cathoderay oscillographs, and mor especially where a balanced output withrespect to the battery busb-ars is required from an unbalanced input ofsteady or alternating nature. An example of such use is the amplifierrequired for feeding a push pull thermionic valve stage.

The use of the amplifier is not limited to the linear portion of thecharacteristic of the valve, provided the valves are substantiallymatched, so that changes of anode current are of similar magnitude. Thestatic neutralizing bias across r3 will still be substantiallyindependent of the changes of anode current. A typical non-linear use ofthe amplifier is to be found in the operation of relays or impulsingcircuits from rectilinear voltage inputs of variable amplitude. Theadvantage to be obtained by employing the amplifier in this manner isthat the current load from the busbars is rendered sensibly constant sothat a power supply having a large internal resistance can be used forinstance a mains rectifier unit.

I claim:

1. A thermionic amplifier for controlling the potential to thedeflecting plates of a cathode ray tube, comprising two thermionic tubeshaving a common cathode resistance, a direct current source of potentialfor energizing said tubes, a connection from the anode of each tube toone of said deflecting plates, the anode of one thermionic tubeconnected through a direct current connection to the grid of the secondthermionic tube by means of a resistance potentiometer connected betweensaid anode and the common cathode resistance with a point thereonconnected to the grid of the second tube, the gain of the firstthermionic tube at the grid of the second tube being unity, and a pairof anode resistances joined together to form a potentiometer with theadjustable point thereof connected to the positive terminal of saidpotential source.

2. A thermionic amplifier as claimed in claim 1 wherein the adjustmentof the center point of the anode resistance potentiometer adjusts theinitial spot position of the cathode ray beam.

3. A thermionic amplifier having a balanced output comprising twothermionic tubes for controlling the potential on the deflecting platesof a cathode ray tube, a direct current source of potential forenergizing said tubes, said thermionic tubes being connected in such amanner that the grid potential of one is controlled by the anodepotential of the other over a direct current path such that the gain ofthe first thermionic tube at the grid of the second tube is unity, aconnection from the anode of each tube to one of the plates of thecathode ray tube, and means for adjusting the initial spot position ofthe cathode ray beam, without altering the potential difference betweenthe plates or the static potential diiierences between the grids of thethermionic tubes, comprising a potentiometer havin its ends connected tothe anodes of the two thermionic tubes and its adjustable pointconnected to the positive terminal of said potential source.

4. A thermionic amplifier for controlling the potentials on thedeflecting plates of a cathode ray tube, comprising two thermionic tubeshaving a common cathode resistance, a connection from the anode of eachtube to one of said plates, a direct current source of potential forenergizing said tubes, a direct current connection from the anode of onethermionic tube to the grid of the second tube such that the ratio ofthe change of voltage applied to the grid of said second tube, 'to thechange of anode voltage of said one tube due to any input voltage, isnominally equal to the inverse of the gain of said one tube, and a pairof anode resistances joined together to form a potentiometer with itsadjustable point connected to the positive terminal of said potentialsource.

BERTRAM MORTON HADFIELD.

